The pill tricks the body into thinking it has consumed a large amount of calories - as if you have just eaten a substantial meal.
The dieter's dream, it would allow them to continue to enjoy fatty foods without putting on weight.
The drug, which tricks the body into believing it has eaten, also cuts cholesterol and keeps blood sugar levels under control.
Researcher Ronald Evans said: 'This pill is like an imaginary meal.
'It sends out the same signals that normally happen when you eat a lot of food, so the body starts clearing out space to store it.
'But there are no calories and no change in appetite.'
British doctors described the US breakthrough as potentially of great importance – but cautioned that much more work needs to be done.
The research comes amid growing concern that the world is losing the battle against obesity.
Obesity has reached epidemic proportions globally, with at least 2.8 million people dying each year as a result of being overweight or obese, according to the World Health Organisation.
It warns 44 per cent of the diabetes burden, 23 per cent of the heart disease burden and between 7 per cent and 41 per cent of the burden of certain cancers are attributable to people being overweight or obese.
And with just one diet drug on the market, growing numbers of people are restoring to going under the surgeon's knife.
The excitement surrounds a new medicine called fexaramine.
Given to obese mice fed fatty food, it stopped them from piling on the pounds.
Fat was burnt off, levels of artery-clogging cholesterol fell and blood sugar levels were better controlled.
The drug also sped up metabolism and turned harmful white fat that wobbles around the waist into healthy brown fat.
